Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A power supply circuit and a method for operating a power supply circuit
involves selecting a normal operational mode or a pass-through
operational mode for a switched mode power supply, in the normal
operational mode, converting an input voltage of a power supply circuit
to an intermediate voltage using a switching regulator of the switched
mode power supply, in the pass-through operational mode, disabling the
switching regulator such that the input voltage of the power supply
circuit is unchanged by the switching regulator and an electric current
consumption of the switching regulator approaches zero, and converting
the intermediate voltage or the input voltage of the power supply circuit
to an output voltage using a linear voltage regulator.

Claims:

1. A method for operating a power supply circuit, the method comprising:
selecting a normal operational mode or a pass-through operational mode
for a switched mode power supply; in the normal operational mode,
converting an input voltage of a power supply circuit to an intermediate
voltage using a switching regulator of the switched mode power supply; in
the pass-through operational mode, disabling the switching regulator such
that the input voltage of the power supply circuit is unchanged by the
switching regulator and an electric current consumption of the switching
regulator approaches zero; and converting the intermediate voltage or the
input voltage of the power supply circuit to an output voltage using a
linear voltage regulator.

2. The method of claim 1, wherein selecting the normal operational mode
or the pass-through operational mode for the switched mode power supply
comprises determining a demand for an output current that flows out of
the switched mode power supply.

3. The method of claim 2, wherein selecting the normal operational mode
or the pass-through operational mode for the switched mode power supply
further comprises: selecting the normal operational mode for the switched
mode power supply if the demand for the output current is determined to
be high; and selecting the pass-through operational mode for the switched
mode power supply if the demand for the output current is determined to
be low.

4. The method of claim 3, wherein selecting the normal operational mode
or the pass-through operational mode for the switched mode power supply
further comprises: enabling a bypass circuit within the switching
regulator if the pass-through operational mode is selected; and disabling
the bypass circuit within the switching regulator if the normal
operational mode is selected.

5. A power supply circuit comprising: a switched mode power supply
comprising: a switching regulator configured to convert an input voltage
of the power supply circuit to an intermediate voltage when the switched
mode power supply is in a normal operational mode and to be disabled such
that the input voltage of the power supply circuit is unchanged by the
switching regulator and an electric current consumption of the switching
regulator approaches zero when the switched mode power supply is in a
pass-through operational mode; and a linear voltage regulator serially
connected to the switched mode power supply, wherein the linear voltage
regulator is configured to convert the intermediate voltage or the input
voltage of the power supply circuit to an output voltage.

6. The power supply circuit of claim 5, wherein the switched mode power
supply further comprises a control circuit configured to select the
normal operational mode or the pass-through operational mode for the
switched mode power supply.

7. The power supply circuit of claim 6, wherein the control circuit is
further configured to determine a demand for an output current that flows
out of the switched mode power supply.

8. The power supply circuit of claim 7, wherein the control circuit is
further configured: to select the normal operational mode for the
switched mode power supply if the demand for the output current is
determined to be high; and to select the pass-through operational mode
for the switched mode power supply if the demand for the output current
is determined to be low.

9. The power supply circuit of claim 6, wherein the switching regulator
comprises a bypass circuit configured to disable the switching regulator
when the switched mode power supply is in the pass-through operational
mode, and wherein the control circuit is further configured: to enable
the bypass circuit if the pass-through operational mode is selected; and
to disable the bypass circuit if the normal operational mode is selected.

10. The power supply circuit of claim 9, wherein the bypass circuit
comprises: a first switch; and a second switch.

11. The power supply circuit of claim 10, wherein the switching regulator
further comprises: an inductor connected between the first switch and the
second switch; and a capacitor.

12. The power supply circuit of claim 11, wherein the switching regulator
further comprises a third switch and a fourth switch, wherein the control
circuit is connected to the first, second, third, and fourth switches,
and wherein the control circuit is further configured: to enable the
first switch and the second switch if the pass-through operational mode
is selected; and to switch the first, second, third, and fourth switches
depending on a difference between the input voltage of the power supply
circuit and a desired intermediate voltage if the normal operational mode
is selected.

13. The power supply circuit of claim 6, wherein the switching regulator
comprises a switch configured to disable the switching regulator when the
switched mode power supply is in the pass-through operational mode, and
wherein the control circuit is further configured: to enable the switch
if the pass-through operational mode is selected; and to disable the
switch if the normal operational mode is selected.

14. The power supply circuit of claim 6, wherein the control circuit is a
digital circuit.

15. The power supply circuit of claim 6, wherein the control circuit is
an analog circuit.

16. The power supply circuit of claim 5, wherein the linear voltage
regulator comprises: a transistor connected to the switched mode power
supply and the output voltage; a comparator, the comparator comprising: a
first input terminal connected to a reference voltage; a second input
terminal; and an output terminal connected to a gate of the transistor; a
first resistor connected to the second terminal of the comparator and to
the output voltage; and a second resistor connected to the second
terminal of the comparator and to ground.

17. A power supply circuit comprising: a switched mode power supply
comprising: a switching regulator configured to convert an input voltage
of the power supply circuit to an intermediate voltage when the switched
mode power supply is in a normal operational mode and to be disabled such
that the input voltage of the power supply circuit is unchanged by the
switching regulator and an electric current consumption of the switching
regulator approaches zero when the switched mode power supply is in a
pass-through operational mode, wherein the switching regulator comprises
a bypass circuit configured to disable the switching regulator when the
switched mode power supply is in the pass-through operational mode; and a
control circuit configured: to determine a demand for an output current
that flows out of the switched mode power supply; to select the normal
operational mode for the switched mode power supply if the demand for the
output current is determined to be high; to select the pass-through
operational mode for the switched mode power supply if the demand for the
output current is determined to be low; to enable the bypass circuit if
the pass-through operational mode is selected; and to disable the bypass
circuit if the normal operational mode is selected; and a linear voltage
regulator serially connected to the switched mode power supply, wherein
the linear voltage regulator is configured to convert the intermediate
voltage or the input voltage of the power supply circuit to an output
voltage.

18. The power supply circuit of claim 17, wherein the bypass circuit
comprises: a first switch; and a second switch, and wherein the switching
regulator further comprises: an inductor connected between the first
switch and the second switch; and a capacitor.

19. The power supply circuit of claim 18, wherein the switching regulator
further comprises a third switch and a fourth switch, wherein the control
circuit is connected to the first, second, third, and fourth switches,
and wherein the control circuit is further configured: to enable the
first switch and the second switch if the pass-through operational mode
is selected; and to switch the first, second, third, and fourth switches
depending on a difference between the input voltage of the power supply
circuit and a desired intermediate voltage if the normal operational mode
is selected.

20. The power supply circuit of claim 17, wherein the linear voltage
regulator comprises: a transistor connected to the switched mode power
supply and the output voltage; a comparator, the comparator comprising: a
first input terminal connected to a reference voltage; a second input
terminal; and an output terminal connected to a gate of the transistor; a
first resistor connected to the second terminal of the comparator and to
the output voltage; and a second resistor connected to the second
terminal of the comparator and to ground.

Description:

[0001] Embodiments of the invention relate generally to electrical systems
and methods and, more particularly, to power supply circuits and methods
for operating a power supply circuit.

[0002] A power supply circuit converts an input voltage to a desired
output voltage. The performance of the power supply circuit can be
determined by evaluating characteristics or parameters of the power
supply circuit, such as standby current consumption, load response, and
noise level.

[0003] A power supply circuit and a method for operating a power supply
circuit involves selecting a normal operational mode or a pass-through
operational mode for a switched mode power supply, in the normal
operational mode, converting an input voltage of a power supply circuit
to an intermediate voltage using a switching regulator of the switched
mode power supply, in the pass-through operational mode, disabling the
switching regulator such that the input voltage of the power supply
circuit is unchanged by the switching regulator and an electric current
consumption of the switching regulator approaches zero, and converting
the intermediate voltage or the input voltage of the power supply circuit
to an output voltage using a linear voltage regulator.

[0004] In an embodiment, a method for operating a power supply circuit
involves selecting a normal operational mode or a pass-through
operational mode for a switched mode power supply, in the normal
operational mode, converting an input voltage of a power supply circuit
to an intermediate voltage using a switching regulator of the switched
mode power supply, in the pass-through operational mode, disabling the
switching regulator such that the input voltage of the power supply
circuit is unchanged by the switching regulator and an electric current
consumption of the switching regulator approaches zero, and converting
the intermediate voltage or the input voltage of the power supply circuit
to an output voltage using a linear voltage regulator.

[0005] In an embodiment, a power supply circuit includes a switched mode
power supply and a linear voltage regulator serially connected to the
switched mode power supply. The switched mode power supply includes a
switching regulator configured to convert an input voltage of the power
supply circuit to an intermediate voltage when the switched mode power
supply is in a normal operational mode and to be disabled such that the
input voltage of the power supply circuit is unchanged by the switching
regulator and an electric current consumption of the switching regulator
approaches zero when the switched mode power supply is in a pass-through
operational mode. The linear voltage regulator is configured to convert
the intermediate voltage or the input voltage of the power supply circuit
to an output voltage.

[0006] In an embodiment, a power supply circuit includes a switched mode
power supply and a linear voltage regulator serially connected to the
switched mode power supply. The switched mode power supply includes a
switching regulator and a control circuit. The switching regulator is
configured to convert an input voltage of the power supply circuit to an
intermediate voltage when the switched mode power supply is in a normal
operational mode and to be disabled such that the input voltage of the
power supply circuit is unchanged by the switching regulator and an
electric current consumption of the switching regulator approaches zero
when the switched mode power supply is in a pass-through operational
mode. The switching regulator includes a bypass circuit configured to
disable the switching regulator when the switched mode power supply is in
the pass-through operational mode. The control circuit is configured to
determine a demand for an output current that flows out of the switched
mode power supply, to select the normal operational mode for the switched
mode power supply if the demand for the output current is determined to
be high, to select the pass-through operational mode for the switched
mode power supply if the demand for the output current is determined to
be low, to enable the bypass circuit if the pass-through operational mode
is selected, and to disable the bypass circuit if the normal operational
mode is selected. The linear voltage regulator is configured to convert
the intermediate voltage or the input voltage of the power supply circuit
to an output voltage.

[0007] Other aspects and advantages of embodiments of the present
invention will become apparent from the following detailed description,
taken in conjunction with the accompanying drawings, depicted by way of
example of the principles of the invention.

[0008]FIG. 1 is a schematic block diagram of a power supply circuit in
accordance with an embodiment of the invention.

[0009] FIG. 2 illustrates a state machine diagram of the switched mode
power supply depicted in FIG. 1.

[0010]FIG. 3 depicts an embodiment of the power supply circuit of FIG. 1.

[0011] FIGS. 4 and 5 depict two embodiments of the linear voltage
regulator of FIGS. 1 and 3.

[0012] FIG. 6 is a process flow diagram of a method for operating a power
supply circuit in accordance with an embodiment of the invention.

[0013] Throughout the description, similar reference numbers may be used
to identify similar elements.

[0014] It will be readily understood that the components of the
embodiments as generally described herein and illustrated in the appended
figures could be arranged and designed in a wide variety of different
configurations. Thus, the following detailed description of various
embodiments, as represented in the figures, is not intended to limit the
scope of the present disclosure, but is merely representative of various
embodiments. While the various aspects of the embodiments are presented
in drawings, the drawings are not necessarily drawn to scale unless
specifically indicated.

[0015] The described embodiments are to be considered in all respects only
as illustrative and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by this detailed
description. All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their scope.

[0016] Reference throughout this specification to features, advantages, or
similar language does not imply that all of the features and advantages
that may be realized with the present invention should be or are in any
single embodiment. Rather, language referring to the features and
advantages is understood to mean that a specific feature, advantage, or
characteristic described in connection with an embodiment is included in
at least one embodiment. Thus, discussions of the features and
advantages, and similar language, throughout this specification may, but
do not necessarily, refer to the same embodiment.

[0017] Furthermore, the described features, advantages, and
characteristics of the invention may be combined in any suitable manner
in one or more embodiments. One skilled in the relevant art will
recognize, in light of the description herein, that the invention can be
practiced without one or more of the specific features or advantages of a
particular embodiment. In other instances, additional features and
advantages may be recognized in certain embodiments that may not be
present in all embodiments of the invention.

[0018] Reference throughout this specification to "one embodiment," "an
embodiment," or similar language means that a particular feature,
structure, or characteristic described in connection with the indicated
embodiment is included in at least one embodiment. Thus, the phrases "in
one embodiment," "in an embodiment," and similar language throughout this
specification may, but do not necessarily, all refer to the same
embodiment.

[0019]FIG. 1 is a schematic block diagram of a power supply circuit 100
in accordance with an embodiment of the invention. The power supply
circuit may be used for various devices and applications, such as
computers, industrial machineries, and household appliances. In some
embodiments, the power supply circuit is used for an automotive
application. For example, the power supply circuit is used in body
controllers, anti-lock braking systems (ABS), electronic stability
program (ESP) braking systems, engine management systems, and gear
control systems of motor vehicles.

[0020] In the embodiment depicted in FIG. 1, the power supply circuit 100
includes a switched mode power supply 102 and a linear voltage regulator
104. The switched mode power supply of the power supply circuit includes
a switching regulator 106 and a control circuit 110. Although the power
supply circuit is depicted and described with certain components and
functionality, other embodiments of the power supply circuit may include
fewer or more components to implement less or more functionality.

[0021] The switched mode power supply 102 is configured to operate in
different operational modes. FIG. 2 illustrates a state machine diagram
of the switched mode power supply depicted in FIG. 1. In the state
machine diagram of FIG. 2, the switched mode power supply is configured
to switch between a normal operational mode 202 and a pass-through
operational mode 204. The pass-through operational mode can also be
referred as a standby operational mode.

[0022] Turning back to FIG. 1, the switching regulator 106 of the switched
mode power supply 102 is configured to convert an input voltage
"Vin" of the power supply circuit 100 to an intermediate voltage
"Vsmps" in the normal operational mode 202 and to be disabled such
that the input voltage Vin of the power supply circuit is unchanged
by the switching regulator and an electric current consumption of the
switching regulator approaches zero in the pass-through operational mode
204. The switching regulator may include circuit elements such as at
least one capacitor and at least one inductor.

[0023] In the normal operational mode 202, the switched mode power supply
102 delivers the intermediate voltage Vsmps, which is different from
the input voltage Vin of the power supply circuit 100, and a high
output current Ismps to the linear voltage regulator 104. The normal
operational mode may be, for example, a buck operational mode or a boost
operational mode. In the buck operational mode, the intermediate voltage
Vsmps is lower than the input voltage Vin of the power supply
circuit. In the boost operational mode, the intermediate voltage
Vsmps is higher than the input voltage Vin of the power supply
circuit. In an embodiment, the switched mode power supply includes a
monitoring module (not shown) configured to monitor the input voltage
Vin and to select between the buck operational mode and the boost
operational mode depending on the relationship between the input voltage
Vin and the desired output voltage level.

[0024] In the embodiment depicted in FIG. 1, the switching regulator 106
includes an internal bypass circuit 108. The bypass circuit is configured
to disable the switching regulator 106 in the pass-through operational
mode 204. The bypass circuit may be implemented as a single switch or
serially connected switches.

[0025] In the pass-through operational mode 204, the switched mode power
supply 102 delivers the input voltage Vin of the power supply
circuit 100 and a low output current Ismps, which is zero or close
to zero, to the linear voltage regulator 104. The current consumption of
the switched mode power supply may be limited to the leakage current of
the switched mode power supply only, which is negligible. The switched
mode power supply and the linear voltage regulator are the major thermal
dissipators of the power supply circuit. Because the switched mode power
supply and the linear voltage regulator are either disabled or carrying a
very low current in the pass-through operational mode, the device
junction temperature of the power supply circuit is low, which minimizes
gate leakage of transistors of the power supply circuit.

[0026] The transition from the pass-through operational mode 204 to the
normal operational mode 202 can increase thermal dissipation in the
linear voltage regulator 104. In order to avoid thermal dissipation
buildup in the linear voltage regulator, the switched mode power supply
102 may re-enter the normal operational mode after operating in the
pass-through operational mode after a short time interval, e.g. below 10
milliseconds.

[0027] The control circuit 110 of the switched mode power supply 102 is
configured to select the normal operational mode 202 or the pass-through
operational mode 204 for the switched mode power supply. In other words,
the control circuit is configured to control mode selection between the
normal operational mode and the pass-through operational mode. The
control circuit may be a digital circuit or an analog circuit. Although
the control circuit is depicted in FIG. 1 as being external to the
switching regulator 106, in other embodiments, the control circuit is
integrated into the switching regulator.

[0028] The control circuit 110 may be connected to the bypass circuit 108
and to the linear voltage regulator 104 and may be further configured to
determine a demand for the output current Ismps that flows out of
the switched mode power supply 102. In the embodiment depicted in FIG. 1,
the output current Ismps flows from the switched mode power supply
to the linear voltage regulator 104. In some embodiments, the power
supply circuit 100 includes one or more additional voltage regulator
and/or load located between the switched mode power supply and the linear
voltage regulator. In this case, the output current Ismps flows from
the switched mode power supply to the additional voltage regulator and/or
load. The control circuit may select the normal operational mode 202 or
the pass-through operational mode 204 for the switched mode power supply
based on the demand for the output current Ismps. For example, the
control circuit selects the normal operational mode if the demand for the
output current Ismps is high and selects the pass-through
operational mode if the demand for the output current Ismps is low.
In an embodiment, the demand for the output current Ismps is
determined to be high if the demanded output current Ismps is around
or above 400 milliamp (mA) and the demand for the output current
Ismps is determined to be low if the demanded output current
Ismps is below 50 mA.

[0029] In the pass-through operational mode 204, the input voltage to the
linear voltage regulator 104 is equal to the input voltage Vin of
the power supply circuit 100. Because the demand for the output current
Ismps is low, the power dissipation in the linear voltage regulator
stays low even with a higher voltage drop across the linear voltage
regulator, compared with the voltage drop in the normal operational mode
202.

[0030] In an embodiment, the bypass circuit 108 includes a switch (not
shown) that is configured to be controlled by the control circuit 110 to
disable the switching regulator 106 in the pass-through operational mode
204. In this case, the control circuit is configured to enable, i.e.,
close the switch if the pass-through operational mode 202 is selected and
to disable, i.e. open the switch if the normal operational mode is
selected.

[0031] The linear voltage regulator 104 of the power supply circuit 100 is
serially connected to the switched mode power supply 102. The linear
voltage regulator may include at least one transistor. For example, the
linear voltage regulator includes a p-channel metal-oxide-semiconductor
(PMOS) transistor or an n-channel metal-oxide-semiconductor (NMOS)
transistor. In the embodiment depicted in FIG. 1, the linear voltage
regulator is configured to convert the intermediate voltage Vsmps or
the input voltage Vin to an output voltage "Vout." In an
embodiment, the linear voltage regulator is a low-dropout linear voltage
regulator with a low output ripple. Such a low-dropout linear voltage
regulator allows the output voltage Vout to serve as a reference
voltage for an analog-to-digital converter (ADC), which may be integrated
into a micro controller or into a digital signal processor (DSP).

[0032] The performance of the power supply circuit 100 can be determined
by evaluating characteristics or parameters of the power supply circuit,
such as standby current consumption, load response, noise level, and
power supply rejection ratio (PSRR). The load response of the power
supply circuit may be defined as the variation of output voltage under
output/load current variations. The standby current of the power supply
circuit may be defined as the current consumption under zero/no demand
for the output current Ismps.

[0033] By including the switching regulator 106with the internal bypass
circuit 108 and the linear voltage regulator 104 that is serially
connected with the switched mode power supply, the power supply circuit
100 in the embodiment depicted in FIG. 1 is optimized for both the
standby current consumption and the load response. Specifically, the
pass-through operational mode 204 of the switched mode power supply
allows for ultra low standby current consumption while the serially
connected linear voltage regulator allows for excellent load response. As
a result, the power supply circuit 100 combines the benefit of ultra low
standby current consumption and the benefit of excellent load response.
In addition, the power supply circuit 100 also exhibits low output
voltage ripple and has a good PSRR.

[0034] Compared with a power supply circuit that includes a switched mode
power supply connected in parallel to a linear voltage regulator, the
power supply circuit 100 in the embodiment depicted in FIG. 1 exhibits
excellent load response. For example, because the output of the switched
mode power supply, which may have excessive ripples, in the parallel
connected power supply circuit is directly taken as the output voltage of
the power supply circuit, the output voltage of the power supply circuit
cannot serve as a reference voltage for an ADC. A more severe drawback of
the parallel connected power supply circuit is the load response
performance. When the switched mode power supply is disabled and load
current jumps from a value close to zero to a high value, the parallel
connected power supply circuit needs to transit between a standby mode,
in which the linear voltage regulator is enabled and the switched mode
power supply is disabled and a normal mode in which the linear voltage
regulator is disabled and the switched mode power supply is enabled. The
time required to transition between the two modes can significantly
decrease the load response performance.

[0035] In addition, compared with a power supply circuit that includes a
serially connected switched mode power supply that includes a switching
regulator that does not have the pass-through operational mode 204, and
linear voltage regulator the power supply circuit 100 in the embodiment
depicted in FIG. 1 exhibits an ultra low standby current consumption.
Specifically, the switched mode power supply 102 of FIG. 1 consumes a
moderate to high amount of standby current. Without the bypass circuit
that can disable the switched mode power supply when the demand for the
output current is low, the standby current consumed by the switched mode
power supply and the linear voltage regulator cannot satisfy the low
consumption requirement.

[0036] Furthermore, compared with a power supply circuit that includes
only the linear voltage regulator, the power supply circuit 100 in the
embodiment depicted in FIG. 1 exhibits better power dissipation. Using
only the linear voltage regulator, even a moderate output current
combined with a large difference between the input voltage and the output
voltage of the linear voltage regulator can quickly result in large power
dissipation. For example, if the linear voltage regulator is located in a
typical automotive power supply, a costly and cumbersome cooling
mechanism is needed to ensure the power dissipation or the desired output
current has to be unnecessarily reduced.

[0037]FIG. 3 depicts an embodiment of the power supply circuit 100 of
FIG. 1. In the embodiment depicted in FIG. 3, the power supply circuit
300 includes a switched mode power supply 302, a linear voltage regulator
104, and a capacitor 320. The switched mode power supply includes a
switching regulator 306 that includes an internal bypass circuit 308, and
a control circuit 310.

[0038] The switching regulator 306 of the switched mode power supply 302
is configured to convert an input voltage Vin to an intermediate
voltage Vsmps in a normal operational mode 202 and to be disabled
such that the input voltage Vin of the power supply circuit 300 is
unchanged by the switching regulator and an electric current consumption
of the switching regulator approaches zero in the pass-through
operational mode 204. In the embodiment depicted in FIG. 3, the switching
regulator also includes an inductor 314, another capacitor 318 connected
to the intermediate voltage Vsmps, and switches 328, 330. In an
embodiment, the inductor 314 and the capacitors 318, 320 are placed
external to the power supply circuit. In an embodiment, the switches 328,
330 are realized by NMOS devices, which require less substrate area
compared with PMOS devices.

[0039] The bypass circuit 308 of the switching regulator 306 is configured
to disable the switching regulator 306 in the pass-through operational
mode 204. In the embodiment depicted in FIG. 3, the bypass circuit
includes a first switch 322 and a second switch 326. As shown in FIG. 3,
the inductor 314 is connected between the first switch 322 and a second
switch 326. In an embodiment, the first switch 322 and the second switch
326 are realized by NMOS devices, which require less substrate area
compared with PMOS devices.

[0040] The control circuit 310 of the switched mode power supply 302 is
configured to select the normal operational mode 202 or the pass-through
operational mode 204 for the switched mode power supply. In the
embodiment depicted in FIG. 3, the control circuit is connected to the
bypass circuit 308 and to the linear voltage regulator 104 and is further
configured to determine the demand for the output current Ismps that
flows out of the switched mode power supply. The control circuit may
select the normal operational mode or the pass-through operational mode
based on the demand for the output current Ismps. For example, the
control circuit selects the normal operational mode if the demand for the
output current Ismps is high and selects the pass-through
operational mode if the demand for the output current Ismps is low.
In the embodiment depicted in FIG. 3, the control circuit is connected to
the switches 322, 326, 328, 330 and is further configured to enable the
first switch 322 and the second switch 326 if the pass- through
operational mode is selected and to operate, i.e., to switch the switches
322, 326, 328, 330 on and off depending on the difference between the
actual input voltage Vin and a desired intermediate voltage
Vsmps if the normal operational mode is selected.

[0041] The linear voltage regulator 104 in the embodiments depicted in
FIGS. 1 and 3 can be implemented using PMOS or NMOS transistor elements.
FIGS. 4 and 5 depict two embodiments of the linear voltage regulator of
FIGS. 1 and 3 with a PMOS transistor or a NMOS transistor.

[0042] In the embodiment depicted in FIG. 4, the linear voltage regulator
404 includes a PMOS transistor 440, a comparator 442, a first resistor
444, and a second resistor 446. The PMOS transistor 440 is connected to
the switched mode power supply 102 or 302 (not shown in FIG. 4) and the
output voltage Vout of the power supply circuit 100 or 300 (not
shown in FIG. 4). The comparator 442 includes an input terminal 448
connected to a reference voltage source 450, a second input terminal 452,
and an output terminal 454 connected to the gate 456 of the PMOS
transistor 440. The first resistor 444 is connected to the second
terminal 452 of the comparator 442 and to the output voltage Vout of
the power supply circuit 100 or 300. The second resistor 446 is connected
to the second terminal 452 of the comparator 442 and ground.

[0043] In the embodiment depicted in FIG. 5, the linear voltage regulator
504 includes an NMOS transistor 540, a comparator 542, a first resistor
544, and a second resistor 546. The NMOS transistor 540 is connected to
the switched mode power supply 102 or 302 (not shown in FIG. 5) and the
output voltage Vout of the power supply circuit 100 or 300 (not
shown in FIG. 5). The comparator 542 includes an input terminal 548
connected to a reference voltage source 550, a second input terminal 552,
and an output terminal 554 connected to the gate 556 of the NMOS
transistor 540. The first resistor 544 is connected to the second
terminal 552 of the comparator 542 and to the output voltage Vout of
the power supply circuit 100 or 300. The second resistor 546 is connected
to the second terminal 552 of the comparator 542 and ground.

[0044] FIG. 6 is a process flow diagram of a method for operating a power
supply circuit in accordance with an embodiment of the invention. At
block 602, a normal operational mode or a pass-through operational mode
is selected for a switched mode power supply. At block 604, in the normal
operational mode, an input voltage of a power supply circuit is converted
to an intermediate voltage using a switching regulator of the switched
mode power supply. At block 606, in the pass-through operational mode,
the switching regulator is disabled such that the input voltage of the
power supply circuit is unchanged by the switching regulator and an
electric current consumption of the switching regulator approaches zero.
At block 608, the intermediate voltage or the input voltage of the power
supply circuit is converted to an output voltage using a linear voltage
regulator.

[0045] Although the operations of the method herein are shown and
described in a particular order, the order of the operations of the
method may be altered so that certain operations may be performed in an
inverse order or so that certain operations may be performed, at least in
part, concurrently with other operations. In another embodiment,
instructions or sub-operations of distinct operations may be implemented
in an intermittent and/or alternating manner.

[0046] In addition, although specific embodiments of the invention that
have been described or depicted include several components described or
depicted herein, other embodiments of the invention may include fewer or
more components to implement less or more feature.

[0047] Furthermore, although specific embodiments of the invention have
been described and depicted, the invention is not to be limited to the
specific forms or arrangements of parts so described and depicted. The
scope of the invention is to be defined by the claims appended hereto and
their equivalents.